TECHNICAL INFORMATION ON BUILDING MATERIALS

TIBM -28

FOR USE IN THE DESIGN OF LOW-COST HOUSING

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THE NATIONAL BUREAU OF STANDARDS

UNITED STATES DEPARTMENT OF COMMERCE

WASHINGTON, D. C.

August o, 195

PAINTING OF FERROUS METALS

The principal advantage of painting iron or steel parts of a house

is to afford protection against corrosion* However, the decorative

effect is of increasing importance with the widening use of steel for

"building purposes* Bare steel surfaces, as well as tin or terne-coated

metal, should be painted without delay after installation.

Galvanized surfaces should not be painted for at least six months

after installation. Weathering roughens a galvanized surface, greatly

improving the adherence of the paint. If, however, it is necessary to

paint such surfaces before they have been allowed to weather, they may

be treated with an etching solution. Several of these, as well as a number of paints for use on various metals, are described in National

Bureau of Standards Letter Circular U22, "The Painting of Structural

Metal (Steel, Galvanized Metal, Tin Plate and Copper)", (August 10,

193^).^ hy Percy H. Walker and E. F„ Hickson.

Before being painted, a metal surface should be freed from scale,

oily or greasy films or rust, and should be dry. Oil or grease is

removed by a solvent such as gasoline or benzene; scale and rust by a wire scratch brush or a sand blast. Painting should be done on a clear

warm day; never in wet or foggy weather. The paint may be applied by

brushing or by spraying.

Paints consist of a liquid portion called the "vehicle" and a solid portion known as the "pigment". The vehicle contains a nonvolatile portion consisting of drying oils such as linseed, fish, tung,

etc., resins and compounds of drying metals, as well as volatile

thinner, such as mineral spirits, turpentine, etc. The pigment consists of very finelv divided solid matter that is essentially insoluble

in the vehicle. Many paint technologists recommend that the pigment in

Available from the National Bureau of Standards, Washington, D. C.

(Free).

OSIU36-C 1

a dried paint film should he at least 22 percent "by volume. Among the

important uigments for paints to "be used on metals are red lead, "basic

lead chromate, iron oxide, white lead, zinc oxide, various carbon pigments, aluminum powder, etc. Under conditions favorable to corrosion

of the "basis metal, such as "bare spots, "breaks in the coating, etc.,

some pigments, such as carbon black, increase corrosion of the metal

if in direct contact with it. Other pigments, such as basic lead

chromate, retard corrosion under similar circumstances. It is important, therefore, in priming or first coats, to avoid the use of

paints containing cafbon black, although this is often an excellent

pigment in a finish paint coat.

The "covering" or hiding power of a paint is largely determined

by the specific properties and particle size of the pigment material.

In general, the finer the particle size, the greater the covering power.

Other important qualities of the pigment are color-permanence and inert- ness to attack by the atmosphere. The deteriorating effect of sunlight

on the vehicle of paints may be minimized by the use of dark pigments

which tend to exclude the harmful radiation. Aluminum paint is considered very good from this standpoint because the aluminum pigment

consists of particles having the form of small thin scales which, by

"leafing out" in the liquid vehicle, overlap and form a layer which

efficiently excludes harmful light rays. Aluminum paint is also

considered relatively effective in excluding water from the metal

surface to which it is applied.

In nearly every case, at least two coats should be used in painting the metal parts of a house. The first or "priming" coat on bare

iron or steel should be a paint containing inhibit ive pigments, and

should cover the metal completely . It should be free from pores or blisters, and should adhere well to the metal surface. After the

first coat has completely dried, the finishing paint should be applied in one or more coats as needed. Each coat should be allowed

to dry thoroughly before the next is applied. More than two finish

coats are not ordinarily required. It is important to avoid building

up paint films to an excessive thickness because of the trouble which

may result from scaling.

Some methods of painting metals to be buried in the soil are suggested in TIBM - 29, "Corrosion of Eerrous Metals Underground,"

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